The “Brillouin effect” is a non linear scattering phenomenon in which incident photons of light interact with mechanical vibrations of the medium inside which they propagates to get scattered with a wavelength shifted with respect to the original one, in which the wavelength shift is related to the electro-optical characteristics of the same medium and to the physical characteristics, among which mechanical strain and temperature, that can later such characteristics.
Due to the small entity of the Brillouin wavelength shift in conventional optical fibres, measuring such parameter requires techniques sophisticated and expensive to be implemented.